Urea CO(NH2)2 is a diamide. It is acidic when in fresh solution and becomes alkaline upon standing. That is because it hydrolyzes to CO2 and ammonia (NH3). Urea forms resins with aldehydes. The most commonly used aldehyde is formaldehyde to produce urea-formaldehyds (UF) resins. These resins find wide use as wood adhesives. At one time, they were used as colorless wood hardeners.
Main disadvantages of UF adhesive resins are the sensitivity of the bond to moisture and the volatility and toxicity of their formaldehyde component. UF resins emit formaldehyde when curing, and can also emit it from a glued product.
Aldehydes besides formaldehyde will react with urea to form resins. Furfural is one of the examples. Furfural has lower vapor pressure than formaldehyde, so there are lower emissions. Furfural-urea resins are dark, crosslinked and waterproof. Furfural and urea are widely found in nature and are cheap commercial chemicals. Furfural is a derivative of pentosans found in plants. Major commercial sources are sugar cane bagasse and agricultural residues.
Maleic anhydride (MA) can be synthesized in high yield from furfural (Sugar series 13 p 225). MA is useful for acidifying resinification mixtures and has the potential of becoming part of the resin. It also can be a coupling agent between resin and substrate. Maleic anhydride is a widely-used industrial chemical. Important uses are in polyesters and plastics compatibilizing agents.
Previous Published Work
There was work done on furfural-urea (FUR) resins in the early 20th century (e.g. p 669-670 of The Chemistry of Synthetic Resins by Carleton Ellis, Reinhold Publishing, 1935). It has been mentioned in another textbook (p119 of Urea-Formaldehyde Resins by Beat Meyer, Addison Wesley Publishing Company, 1979).
FUR resins were mentioned as wood stabilization compounds in a 1960 article, but no data on properties or methods of preparation were given (Weaver, J W, J F Neilson and L S. Goldstein. 1960. Dimensional stabilization of wood with aldehydes and related compounds. FPJ June pp 306-310).
Therefore, a need still exists for an efficient and economical method of producing resins and adhesives based on furfural and urea.